The invention relates to an electronic circuit for detection of direct-sequence, spread-spectrum signals (hereinafter called “DSSS signals”) having frequency spectra which are symmetrical about the suppressed carrier frequency.
Symmetric DSSS signals are among those used for communication purposes. They may be produced by modulating a narrow band signal by binary phase shift keying (PSK) in accordance with a pseudo-random binary sequence. This produces a wide-band, suppressed carrier, double side-band signal. Detection of such a signal is extremely difficult when it is obscured by noise.
Applicant is aware of U.S. patent application Ser. No. 445,646 dated the 30th of Nov. 1982, (Van Etten), an abstract of which was made available to the public after Applicant's priority date in Profile 1643/04/0021 (GRA Vol 83 Issue 12: Accession No AD-D010 037(0). Van Etten describes a technique for detecting the amplitude modulation of spread-spectrum signals by compressing the signal into a narrower bandwidth. The signal is divided between two paths and mixed with an upper sideband signal in one path and with a lower sideband signal in the other path. The upper and lower sideband signals are the sum and difference frequencies of two local oscillators, the first of which produces a signal estimated to have the same frequency as the carrier signal of interest, ie it is the listening frequency. The second local oscillator signal has a frequency higher than the bandwidth of the listening frequency. The two signals produced by mixing with the upper and lower sideband signals are mixed together. This further mixing produces an output signal which consists of the square of the amplitude modulation of the original carrier signal modulated at twice the frequency of the second local oscillator signal. Any offset between the listening and carrier frequencies disappears, and so also does any phase or frequency modulation of the carrier. However, the output signal is independent of the carrier frequency provided that the latter is within the system bandwidth, and accordingly the carrier frequency is not detected.
It is an object of the present inventions to provide a detector for determining the carrier frequency of symmetrical DSSS signals.